Method and apparatus for stopping reaction in a gas phase polymerization reactor system

ABSTRACT

A method of terminating, under emergency conditions, an olefin polymerization reaction conducted in the presence of a transitioned metal based catalyst in a gas phase reactor by using a recycle gas from the reactor or a feed stream to the reactor to operate an expander associated with a circulator which maintains fluidized conditions in the reactor while introducing a kill gas into the reactor to terminate the reaction.

This is a divisional of application Ser. No. 07/827,649, filed Jan. 29,1992, now U.S. Pat. No. 5,270,408.

BACKGROUND OF THE INVENTION

This invention relates to a method for terminating a reaction in a gasphase polymerization system.

It is of course well known that a significant portion of alpha-olefinhomopolymer or copolymer, e.g., ethylene copolymer polymerizations arepresently being conducted in fluidized bed reactors. Karol, et al. U.S.Pat. No. 4,302,566 discloses a typical fluidized bed reactor which ispresently used for the preparation of ethylene copolymers in a fluidizedbed reactor. As suggested in this patent, it is essential to operate thefluid bed reactor at a temperature below the sintering temperature ofthe polymer particles (Note column 12, lines 39-53 of Karol et al. U.S.Pat. No. 4,302,566).

In normal operation, the temperature of the fluidized bed is primarilycontrolled by passing recycle gas through a circulator e.g., acompressor and then through a heat exchanger, wherein the recycle gas iscooled to remove the heat of reaction before it is returned to thefluidized bed (Note column 11, lines 35-53).

If the circulator in the fluidized bed arrangement fails, e.g., due toelectrical or mechanical failure, the cooling means for controlling thetemperature in the bed becomes inoperative. Since olefin reactants arestill in contact with active catalyst, exothermic heat of reactioncauses the temperature of the bed to climb toward sintering temperaturesin a run-away fashion.

Unfortunately, however, allowing the polymerization reaction to continueduring emergency situations could cause severe future operatingproblems. In a gas phase fluidized bed polymerization reaction system,loss of fluidizing gas flow with continued reaction normally results inmelting of the polymer powder and the formation of a molten chunk. Theworst case would be caused by a complete loss of utilities--electricpower, cooling water, instrument air, steam, etc.

As a result, when necessary, the art has resorted to various techniquesfor terminating a fluidized bed polymerization in the shortest possibletime. Thus in one such technique, Stevens et al. U.S. Pat. No. 4,326,048describes a method for terminating a gas phase olefin polymerization byinjecting a carbon oxide. The injection of the carbon oxide may takeplace downstream of the polymerization reactor, e.g., in the recycle gasline (Note Column 4 lines 29-33 of the Stevens et al. U.S. Pat. No.4,326,048).

In another technique, Charsley U.S. Pat. No. 4,306,044 discloses a meansfor introducing carbon dioxide into a gas-phase olefin polymerizationsystem to at least reduce the rate of polymerization. For example, thecarbon dioxide can be injected manually when the polymerization does notrespond to other means of control. One other means of control is byrapid venting of the reactor as fast as possible in an attempt tocontrol the run-away reaction. In this regard it is noted that thefluidized bed system as described in Karol et al. U.S. Pat. No.4,302,566 is expressly provided with a venting system for shutdown.Venting systems of the prior art are relatively large and createadditional problems.

Accordingly, these remedies are not entirely satisfactory because ofeconomic and environmental considerations. Hence, there is still a needin the art for killing a reaction by techniques which do not require alarge venting system.

SUMMARY OF THE INVENTION

In a gas phase fluidized bed polymerization reaction system, loss offluidizing gas flow with continued reaction can result in melting of thepolymer powder and the formation of a molten chunk. The worst case wouldbe caused by a complete loss of utilities--electric power, coolingwater, instrument air, steam, etc. As mentioned previously in the past,fluidized bed reactors have been "killed" by injecting a poison e.g.carbon monoxide and depressuring the system rapidly to a flare or to theatmosphere. A new system is desired which avoids the need for rapidreactor blowdown.

The present invention provides a method for stopping or "killing" apolymerization reaction by providing an auxiliary means for continuingrotation of the cycle gas circulator after a power failure forsufficient time to circulate a catalyst poison throughout the fluidizedbed. The invention preferably utilizes gas driven auxiliary means, e.g.,an expander for the cycle gas circulator utilizing either gas from thereactor itself or gaseous monomer from the reactor feed system to drivethe expander in case of loss of electric power.

In a broad aspect therefore, the present invention provides a method ofterminating, under emergency conditions, an olefin polymerizationreaction conducted in the presence of a transition metal-based catalystin a gas phase reactor wherein a monomer feed stream is introduced intosaid reactor, a cycle gas comprising monomer, comonomer, hydrogen anddiluents is discharged from said reactor and directed through acirculator and at least one heat exchanger, and thereafter returningsaid cooled recycle gas to said reactor, the improvement in the methodof terminating said olefin polymerization reaction which comprisesintroducing a kill gas into said reactor in an amount sufficient toterminate the reaction in said reactor, directing at least a portion ofsaid recycle gas from said reactor or said feed stream, throughauxiliary driving means associated with said circulator to maintainoperation of said circulator at a level such as to maintain fluidizedconditions in said reactor, said kill gas being directed through saidreactor for a period of time sufficient to insure that said kill gascomes into contact with substantially all of said catalyst in saidreactor.

The present invention also provides an apparatus for terminating apolymerization reaction in fluidized bed polymerization reaction systemincluding a polymerization reactor, circulator means for circulating arecycle stream from said reactor and at least one cooling means forcooling said recycle stream, said apparatus comprising kill gas entrymeans for delivering a kill gas to said reactor; gas driven auxiliarymeans associated with said circulator for maintaining operation of saidcirculator during power failures, and valve means associated with saidgas driven auxiliary means for selectively directing said gas to saidgas driven auxiliary means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment for practicingthe method of the present invention.

FIG. 2 is a schematic representation of a seconded embodiment forpracticing the method of the present invention.

DETAILED DESCRIPTION

The invention can be practiced in connection with any process typicallyemployed to produce olefins, e.g., ethylene or propylene homopolymers orcopolymers by a gas fluidized bed reactor process. Merely asillustrative, the fluidized bed reactor can be as described in U.S. Pat.Nos. 4,482,687, or 4,302,566 or another conventional reactor for the gasphase production of, for example, polyethylene, polypropylene orethylene copolymers and terpolymers provided they have been modifiedsuch as shown in FIGS. 1 and 2. The bed is usually made up of the samegranular resin that is to be produced in the reactor. Thus, during thecourse of the polymerization, the bed comprises formed polymerparticles, growing polymer particles, and catalyst particles fluidizedby polymerizable and modifying gaseous components introduced at a flowrate or velocity sufficient to cause the particles to separate and actas a fluid. The fluidizing gas is made up of the initial feed, make-upfeed, and cycle (recycle) gas, i.e., monomer and, if desired, modifiersand/or an inert carrier gas and/or induced condensing agent. A typicalcycle gas is comprised of ethylene, nitrogen, hydrogen, and propylene,butene, hexene monomers or dienes, either alone or in combination.

OPERATING EQUIPMENT

The equipment which can be utilized is basically the same equipment usedin conventional fluidized bed polymerization systems. Thus the processutilizes a conventional fluidized bed polymerization reactor, withappropriate recycle, feed, and catalyst lines, conventional heatexchangers and a compressor or circulator. The circulator however isoperatively associated with an auxiliary driving means adapted tomaintain operation of the circulator during power failure. The auxiliarydriving means receives input power from a pressurized gas which can bethe recycle gas from the reactor or alternatively from the pressurizedmonomer feed gas. The auxiliary means is preferably an expander which isoperatively connected to the circulator through a motor. The expander isconventional in the art and can be obtained from a variety of commercialsources. The equipment also includes strategically positioned controldevices such as valves which regulate the gas input to the circulatorand the reactor. FIG. 1 indicates one technique for carrying out themethod of the present invention. Referring to FIG. 1 there isillustrated a conventional fluidized bed reaction system forpolymerizing alpha olefins which includes a reactor 10 which consists ofa reaction zone 12 and a velocity reduction zone 14.

The reaction zone 12 includes a bed of growing polymer particles, formedpolymer particles and a minor amount of catalyst particles fluidized bythe continuous flow of polymerizable and modifying gaseous components inthe form of make-up feed and recycle gas through the reaction zone. Tomaintain a viable fluidized bed, the mass gas flow rate through the bedis normally maintained above the minimum flow required for fluidization,and preferably from about 1.5 to about 10 tim G_(mf) and more preferablyfrom about 3 to about 6 times G_(mf). G_(mf) is used in the acceptedform as the abbreviation for the minimum gas flow required to achievefluidization, C. Y. Wen and Y. H. Yu, "Mechanics of Fluidization",Chemical Engineering Progress Symposium Series, Vol. 62, p.100-111(1966).

It is highly desirable that the bed always contains particles to preventthe formation of localized "hot spots" and to entrap and distribute theparticulate catalyst throughout the reaction zone. On start up, thereactor is usually charged with a base of particulate polymer particlesbefore gas flow is initiated. Such particles may be identical in natureto the polymer to be formed or different therefrom. When different, theyare withdrawn with the desired formed polymer articles as the firstproduct. Eventually, a fluidized bed of the desired polymer particlessupplants the start up bed.

The appropriate catalyst used in the fluidized bed is preferably storedfor service in a reservoir 16 under a blanket of a gas which is inert tothe stored material, such as nitrogen or argon.

Fluidization is achieved by a high rate of gas recycle to and throughthe bed, typically in the order of about 50 times the rate of feed ofmake up gas. The fluidized bed has the general appearance of a densemass of viable particles in possible free-vortex flow as created by thepercolation of gas through the bed. The pressure drop through the bed isegual to or slightly greater than the mass of the bed divided by thecross-sectional area. It is thus dependent on the geometry of thereactor.

Make-up gas or liquid is fed to the bed at a rate approximately equal tothe rate at which particulate polymer product is withdrawn. A gasanalyzer 18 is positioned above the bed. The gas analyzer determines thecomposition of the gas being recycled and the composition of the make-upgas and/or liquid is adjusted accordingly to maintain an essentiallysteady state gaseous composition within the reaction zone.

To insure complete fluidization, the recycle gas and, where desired,part or all of the make-up gas and/or liquid are returned to the reactorat base 20 below the bed. Gas distribution plate 22 positioned above thepoint of return ensures proper gas distribution and also supports theresin bed when gas flow is stopped.

The portion of the gas stream which does not react in the bedconstitutes the recycle gas which is removed from the polymerizationzone, preferably by passing it into velocity reduction zone 14 above thebed where entrained particles are given an opportunity to drop back into the bed.

The recycle gas is then circulated by circulator 24 and passed through aheat exchanger 26 wherein it is cooled to remove the heat of reactionbefore it is returned to the bed. By constantly removing heat ofreaction, no noticeable temperature gradient appears to exist within theupper portion of the bed. A temperature gradient will exist in thebottom of the bed in a layer of about 6 to 26 inches, between thetemperature of the inlet gas and the temperature of the remainder of thebed. Thus, it has been observed that the bed acts to almost immediatelyadjust the temperature of the recycle gas above this bottom layer of thebed zone to make it conform to the temperature of the remainder of thebed thereby maintaining itself at an essentially constant temperatureunder steady conditions. The recycle is then returned to the reactor atits base 20 and to the fluidized bed through distribution plate 22. Thecirculator 24 can also be placed downstream of heat exchanger 26.

Hydrogen may be used as a chain transfer agent for conventionalpolymerization reactions of the types contemplated herein. In the casewhere ethylene is used as a monomer the ratio of hydrogen/ethyleneemployed will vary between about 0 to about 2.0 moles of hydrogen permole of the monomer in the gas stream.

The hydrogen, nitrogen monomer and comonomer feedstream (gas feed) areintroduced into line 29 through line 28 where it then enters the bottomof reactor 10.

Any gas inert to the catalyst and reactants can also be present in thegas stream. The cocatalyst is added to the gas recycle stream upstreamof its connection with the reactor as from dispenser 30 through line 32.

As is well known, it is essential to operate the fluid bed reactor at atemperature below the sintering temperature of the polymer particles.Thus to insure that sintering will not occur, operating temperaturesbelow sintering temperatures are desired. For example, for theproduction of ethylene polymers an operating temperature of from about90° C. to 100° C. is preferably used to prepare products having adensity of about 0.94 to 0.97 while a temperature of about 75° C. to 95°C. is preferred for products having a density of about 0.91 to 0.94.

Normally the fluid bed reactor is operated at pressures of up to about1000 psi, and is preferably operated at a pressure of from about 150 to550 psi, with operation at the higher pressures in such ranges favoringheat transfer since an increase in pressure increases the unit volumeheat capacity of the gas.

The catalyst is injected into the bed at a rate equal to its consumptionat a point 34 which is above the distribution plate 22. A gas which isinert to the catalyst such as nitrogen or argon is used to carry thecatalyst into the bed. Injecting the catalyst at a point abovedistribution plate 22 is an important feature. Since the catalystsnormally used are highly active, injection into the area below thedistribution plate may cause polymerization to begin there andeventually cause plugging of the distribution plate. Injection into theviable bed, instead, aids in distributing the catalyst throughout thebed and tends to preclude the formation of localized spots of highcatalyst concentration which may result in the formation of "hot spots".

Under a given set of operating conditions, the fluidized bed ismaintained at essentially a constant height by withdrawing a portion ofthe bed as product at a rate equal to the rate of formation of theparticular polymer product. Since the rate of heat generation isdirectly related to product formation, a measurement of the temperaturerise of the gas across the reactor (the difference between inlet gastemperature and exist gas temperature) is determinative of the rate ofthe particulate polymer formation at a constant gas velocity.

The particulate polymer product is preferably withdrawn at a point 36 ator close to distribution plate 22. The particulate polymer product isconveniently and preferably withdrawn through the sequential operationof a pair of timed valves 38 and 40 defining a segregation zone 42.While valve 40 is closed, valve 38 is opened to emit a plug of gas andproduct to the zone 42 between it and valve 38 which is then closed.Valve 40 is then opened to deliver the product to an external recoveryzone and after delivery, valve 40 is then closed to await the nextproduct recovery operation.

Finally, the fluidized bed reactor is equipped with an adequate ventingsystem (although significantly smaller than prior art systems) to allowventing the bed during the start up and shut down. The reactor does notrequire the use of stirring means and/or wall scraping means.

The reactor vessel is normally constructed of carbon steel and isdesigned for the operating conditions stated above.

Modifications to the conventional system are required when it is desiredto terminate the reaction during power failures and other typeemergencies. Thus, referring again to FIG. 1, it will be seen thatcirculator 24 which is driven by double extended shaft motor 44 isassociated with expander 46 in a manner such that rotation of doubleextended shaft 48 will cause operation of circulator 24 even when motor44 is not operating. This is permitted by coupling shaft 48 to couplers49, 50 which couple shaft 48 to shaft 51 of circulator 24 and shaft 48to shaft 53 of expander 46. Expander 46 receives its driving force fromrecycle gas which is directed into bypass recycle gas line 52 in whichis positioned a valve 54. When valve 54 is opened it will be seen thatat least a portion of recycle gas will flow into expander 46 to serve asa driving force. The flow of gas through expander 46 provides the powerrequired to keep circulator 24 and motor 44 running at speeds sufficientto maintain fluidization. Gas circulation is maintained through thereaction system, and the fluidized bed 12 is maintained in a fluidizedstate. The gas which is used to drive expander 46 passes through line56, leaving the reaction system at relatively low pressure. Normally,this is a relatively small gas flow, and it can be disposed of in aconventional flare system or other means of disposal.

The modified reaction system also includes a means for introducing akill gas to the system. Thus referring again to FIG. 1, a kill gas suchas a carbon oxide gas preferably carbon monoxide can be introduced intoreactor 10 by opening valve 58 which permits the kill gas to flowthrough line 60 and eventually into the bottom portion of reactor 10.Line 60 may alternately connect directly to reactor 10 or to some otherportion of the reaction system. By this arrangement it is evident thatthe gas circulation maintained by the action of expander 46 carries thecatalyst poison into the fluidized bed where it stops the polymerizationreaction.

In an alternate technique the driving force for expander 46 can be thefeed gas for the reaction system. Thus referring to FIG. 2, wherein likeparts are designated by like reference numerals monomer feed gas line 28is provided with a valve 68 which controls the flow of feed gas to thereactor. Feed bypass line 62 having valve 64 controls the flow of feedgas to expander 46. When valve 68 is opened and valve 64 is closed,conventional polymerization takes place. However, when valve 68 isclosed and valve 64 is open, the feed gas is directed through lines 62and 66 into expander 46. Operation then beomes similar to the operationdescribed in FIG. 1.

In a typical mode of operation and with reference to FIG. 1, when it isnecessary to stop or "kill" the reaction, valve 54 is opened allowinggas from the reaction system to flow from line 52 through line 54 toexpander 46. The flow of gas through the expander provides the powerrequired to keep circulator 24 and motor 44 running at some reducedspeed, usually about half the normal operating speed. Gas circulation ismaintained through the reaction system, and the fluidized bed 12 ismaintained in a fluidized state. The gas which is used to drive expander46 passes through line 56, leaving the reaction system at relatively lowpressure. Normally, this is a relatively small gas flow, and it can bedisposed of in a conventional flare system or other means of disposal.

At essentially the same time that valve 54 is opened, valve 58 is alsoopened, allowing a gas or liquid which is a catalyst poison, such ascarbon monoxide or carbon dioxide, to flow through line 60 and into line29. The gas circulation maintained by the action of the expander carriesthe catalyst poison into the fluidized bed where it stops thepolymerization reaction.

The following Examples will illustrate the present invention:

EXAMPLE 1

A conventional gas phase fluidized bed polyethylene reaction system suchas disclosed in U.S. Pat. Nos. 4,482,687 or 4,302,566 is utilized andwhich is modified as shown in FIG. 1. The system contains a total gasvolume of about 19,400 cubic feet.

The reactor is operating at a temperature of about 89° C. and isproducing a polymer product which will begin to agglomerate if the bedtemperature exceeds a temperature of about 102° C. Superficial gasvelocity in the reactor is 2.0 ft/sec. The cycle gas circulator isequipped with a supplemental gas expander drive which uses gas from thereactor. In an electric power failure, a valve opens to start gas flowat a rate sufficient to maintain fluidization through the expander,e.g., about 25,000 lb/hr through the expander. The exhaust gas from theexpander is routed to a small flare system which is used to dispose ofmiscellaneous process vents. The expander keeps the circulator operatingat about half its normal speed, approximately 18000 RPM. Carbon monoxideis fed to the reaction system and is circulated through the bed withinapproximately 1 minute, completely stopping all reaction. The maximumtemperature reached in any part of the bed is approximately 96° C.

COMPARATIVE EXAMPLE I

A gas phase fluidized bed reaction system identical to the one describedabove is utilized except it is not equipped with a gas expander.Instead, a large reactor vent line equipped with a shutoff valve isattached to the top of the reaction vessel and is routed to a large,dedicated flare system. A shutoff valve is provided in the cycle gasline between the reactor and the circulator. The reaction system isoperating under conditions identical to those described in the foregoingexample. In a power failure, the valve in the cycle gas line is closed,and the valve in the reactor vent line is opened. At the same time,carbon monoxide is injected into the bottom of the reactor below thefluidized bed. Gas is vented to the flare system at a flow rate ofapproximately 300,000 to 400,000 lb/hr. After approximately two minutes,the carbon monoxide has been distributed throughout the polymer bed, andall reaction has stopped. The maximum temperature reached in any part ofthe bed is approximately 102° C.

As will be seen from Comparative Example I, the gas flow to the flaresystem is at a factor of 10 greater than that required by the expander,and the rate greatly exceeds the capacity of the small flare systemnormally provided for disposal of miscellaneous process vent. Thus, aspecial, dedicated flare system is required so that the vented gas canbe disposed of safely and in an environmentally acceptable manner.

What is claimed is:
 1. A fluidized bed polymerization reaction systemcomprising a polymerization reactor, feeding means for introducing afeed gas into the reactor, first and second circulator means forcirculating a recycle gas from and to said reactor, said firstcirculator means comprising a circulator driven by a motor, at least onecooling means for cooling said recycle gas, a kill gas entry means fordelivering a kill gas to the reactor, gas driven second circulator meansconnected to said first circulator means for maintaining operation ofsaid first circulator means during power failure, and valving meansconnected to said gas driven second circulator means for selectivelydirecting said feed gas or recycle gas to said gas driven secondcirculator means.
 2. Apparatus according to claim 1 wherein said gasdriven second circulator means includes an expander.